The present invention relates to the control of a transfer bias of an image forming apparatus.
In an image forming apparatus of an electrophotographic system, when a toner image is transferred to a sheet (recording sheet) from an image carrier, the sheet is nipped between the image carrier and a transfer member, and a transfer bias is applied between the image carrier and the transfer member. At this time, to obtain a satisfactory transfer image, the magnitude of the transfer bias is important. If, for instance, the transfer bias is insufficient, toner remains on the image carrier without being transferred, and the adhesiveness of the toner on the sheet becomes insufficient, possibly resulting in the scattering of the toner. Conversely, if the transfer bias becomes excessive, an electric charge occurs between the image carrier and the transfer member, which can cause image defects to occur and can also damage the image carrier.
The appropriate magnitude of the transfer bias is affected by the ambient environment, particularly the temperature and humidity. This is because the resistance of the sheet changes based on the temperature and humidity. Accordingly, in a related apparatus, a sheet is subjected to a test flow before the start of an actual copying operation, and during the test flow a volume resistance value between the adjacent transfer roller, sheet, and photoconductor is measured when the sheet is nipped between the photoconductor and the transfer roller, and the transfer bias is controlled based on the measured resistance values. Further, another related apparatus, an electrical resistance value is detected at a stage prior to the transfer, and a transfer current is selected on the basis of this electrical resistance value.